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Analysis of CCN activity of Arctic aerosol and Canadian biomass burning during summer 2008

by T. L. Lathem, A. J. Beyersdorf, K. L. Thornhill, E. L. Winstead, M. J. Cubison, A. Hecobian, J. L. Jimenez, R. J. Weber, B. E. Anderson, A. Nenes show all authors
Atmospheric Chemistry and Physics Discussions ()

Abstract

TheNASA DC-8 aircraft characterized the aerosol properties, chemical composition, and cloud condensation nuclei (CCN) concentrations of the summertime Arctic dur- ing the 2008 NASA Arctic Research of the Composition of the Troposphere from Aircraft and Satellites (ARC- TAS) campaign. Air masses characteristic of fresh and aged biomass burning, boreal forest, Arctic background, and an- thropogenic industrial pollution were sampled. Observations were spatially extensive (50–85◦ N and 40–130◦ W) and ex- hibit significant variability in aerosol and CCN concentra- tions. The chemical composition was dominated by highly oxidized organics (66–94%by volume), with awater-soluble mass fraction of more than 50 %. The aerosol hygroscopicity parameter, κ, ranged between κ =0.08–0.32 for all air mass types. Industrial pollution had the lowest κ of 0.08±0.01, while the Arctic background had the highest and most vari- able κ of 0.32±0.21, resulting from a lower and more vari- able organic fraction. Both fresh and aged (long-range trans- ported) biomass burning air masses exhibited remarkably similar κ (0.18±0.13), consistent with observed rapid chem- ical and physical aging of smoke emissions in the atmo- sphere, even in the vicinity of fresh fires. The organic hygro- scopicity (κorg) was parameterized by the volume fraction of water-soluble organic matter (εWSOM), with a κ =0.12, such that κorg =0.12εWSOM. Assuming bulk (size-independent) ean Geosciences Union. The Cryosphere Earth System Dynamics composition and including the κorg parameterization enabled CCN predictions to within 30% accuracy for nearly all en- vironments sampled. The only exception was for industrial pollution from Canadian oil sands exploration, where an ex- ternal mixture and size-dependent compositionwas required. Aerosol mixing state assumptions (internal vs. external) in all other environments did not significantly affect CCN predic- tions; however, the external mixing assumption provided the best results, even though the available observations could not determine the true degree of external mixing and therefore may not always be representative of the environments sam- pled. No correlation was observed between κorg and O: C. A novel correction of the CCN instrument supersaturation for water vapor depletion, resulting from high concentrations of CCN, was also employed. This correction was especially important for fresh biomass burning plumes where concen-  Instrumentation Methods and Data Systems  Model Development Hydrology and Earth System Sciences trations exceeded 1.5×104 cm−3 and introduced supersat- uration depletions of ≥25 %. Not accounting for supersat- uration depletion in these high concentration environments Ocean Science would therefore bias CCN closure up to 25% and inferred κ by up to 50 %. Solid Earth M cess Open Access Open Access Open Access Open Access Open Access Open Access Open Access Open Access Open Access

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